The use of hydraulic tappets or lash adjusters in the valve train of internal combustion engines is well known. Generally, the function of such tappets is to reduce engine noise due to the looseness associated with various mechanical components of the engine valve train and, also, to eliminate valve tapping that develops as the various parts of the valve train wear in use. Valve tappets are located between the valve rocker arm and the push rod for the valve rocker arm, if the engine is a valve-in-head type, or between the cam shaft and the end of the valve stem, if the engine is the so called "flat head" type. The former arrangement is disclosed in U.S. Pat. No. 3,786,792, while the latter type engine is illustrated in U.S. Pat. No. 2,614,547. Essentially, the valve tappet is comprised of an open-ended cylindrical cup and a plunger telescopically engaging the interior of the cylindrical cup to form a tappet chamber to receive a fluid, usually lubricating oil from the engine's lubrication system. The cup and plunger can move axially relative to one another so that, when the lubricating oil under pressure is admitted into the chamber, the tappet elongates axially and thus causes the slack in the valve train to be eliminated. The oil generally enters the tappet chamber from a suitable port provided for this purpose in the engine block layout passing therein through a spring operated, one-way check valve which blocks the flow of oil out of the tappet chamber, which then remains full, assuming there is no oil seepage from the chamber either past the check valve or from between the opposed telescoped parts.
In recent years, there has been considerable development work done to utilize the internal combustion engine in over the road vehicles not only as a power source for vehicles, but also as a brake to supplement the usual vehicle wheel braking system. Basically, engine braking may occur in either of two ways, that is, "suction" braking or "compression" braking. "Suction" braking inherently occurs in a carburetor type engine equipped with a throttle valve in the intake system whenever the throttle is fully closed because the entire intake system operates at a negative pressure. However, the amount of "suction" braking thus derived is limited by the pressure difference that can be drawn across the closed throttle valve. Obviously, this pressure could never exceed one atmosphere and thus "suction" braking may not be sufficient to provide effective engine "braking" where the engine is used to power a large vehicle. Moreover, those vehicles equipped with engines which have no throttle valve in the intake system as described above, such as a fuel injected or compression ignition engine (e.g., a diesel engine) do not experience an inherent "braking" effect when the operator releases the accelerator pedal because a throttle valve of the type described above is totally lacking. In circumstances where inherent "vacuum" braking is insufficient or non-existent, a different type of engine braking, termed "compression" braking, has been employed which involves the steps of maximizing the intake of air (without admixture of fuel) during the intake stroke, compressing this air during the compression stroke and releasing the compressed air at the end of the compression stroke. To realize the greatest possible engine compression braking, it is essential that the maximum volume of air be drawn into the engine cylinders for compression as the pistons come up on the compression stroke and that this compressed volume be released at the beginning of the expansion stroke.
Engines equipped with throttles in the intake system can be modified to accommodate compression braking as described by using an auxiliary valve in the intake system that would allow air to by-pass the carburetor when in the braking mode, thus preventing introduction of fuel into the cylinders during braking. In a fuel injection or compression ignition engine normal engine operation prevents introduction of fuel during the braking mode; therefore, no auxiliary valve is required. In both throttle equipped engines and in fuel injected or compression ignition engines, the exhaust and intake valves are normally held closed during the last portion of the compression stroke and the beginning of the expansion (firing) strokes when such engines are operated in the power mode. Thus the normal opening and closing of the valves during the power cycle in the operation of an internal combustion engine is at variance with that required should it be desired to operate the engine to effect "compression" braking. The net result is that either the cylinders must be provided with multiple intake and exhaust valves operated by individual selectively operable cam shafts, one such valve system being designed for normal engine operation, the other system being designed to provide engine braking, or, alternatively, and preferably, some mechanism should be provided to operate the usual intake and exhaust valves in one mode for normal (power) engine operation and another mode for engine braking. The obvious benefit of such dual mode operation of the valves is the elimination of the cost and complication of duplicate valve systems or trains.
One method of providing for dual mode operation of the valves is by means of what is known as a dual ramp camshaft, such as illustrated in U.S. Pat. No. 3,786,792, wherein the valve train for each exhaust valve is provided with a cam having a secondary base circle (recessed below the radius of the valve closing, primary base circle) for engaging the valve train during the intake and compression strokes of the associated piston and a hydraulically adjustable tappet selectively operable to cause the exhaust valve to partially open during the expansion stroke of the piston. More particularly the adjustable tappet is spring biased to extend when the valve train is in contact with the secondary base circle of the cam and normally to collapse when the train is in contact with the primary base circle except when hydraulically locked in the extended condition to cause partial opening of the exhaust valve during the expansion stroke as described fully in U.S. Pat. No. 3,786,792. This is a very practical and reasonable approach because the valve train mechanism is not substantially changed from the conventional configuration; there is no expensive duplication of parts; and it is a fairly straightforward uncomplicated matter to provide the hydraulic circuit necessary to selectively lock the adjustable tappet in the extended position. However, in some engine environments the provision of a dual ramp camshaft gives rise to complications especially with regard to valve and piston clearance. For example piston clearance is particularly acute in the case of diesel engines of the overhead valve configuration because the high compression required to cause compression ignition of the diesel fuel in the fuel-air charge, generally on the order of 17 to 1, requires that the piston move very close to the top of the cylinder at its top dead center position. In fact, in most diesel engines, when the piston is in the top dead center position its top face is so close to the cylinder head that, in valve-in-head engines, the valves must be closed to avoid being struck by the piston. Since during the power mode operation of an engine the valves are closed in all cases where the piston reaches top dead center the clearance between pistons and valves is perfectly satisfactory. However, when such an engine is operated in the braking mode the valve timing is changed thereby increasing the possibility that the valves may strike the piston when the piston comes up to top dead center. This problem is of particular concern when the adjustable tappet is continually locked in the elongated condition during the "braking" mode as in the case with the prior art dual ramp cam and tappet arrangement described above since the exhaust valve is raised above its normal fully open position.
In addition to the clearance problem, prior art systems employing hydraulically operated tappets have tended to be complicated, expensive and difficult to retro-fit in an engine. In particular, designs are known for bleeding small amounts of fluid from the tappet chamber of a hydraulic tappet upon preselected displacement of the tappet such as illustrated in U.S. Pat. No. 3,650,251 and designs are known to hydraulically limit the maximum opening displacement of a valve in an engine braking system such as illustrated in U.S. Pat. No. 3,405,699. However, such systems have failed to incorporate the advantages of a dual ramp cam configuration. Moreover, these systems certainly have not suggested a technique for insuring valve clearance while also simplifying the basic hydraulically adjustable tappet designs known heretofor for use with dual ramp cams.